a) Field of the Invention
The present invention is generally concerned with power transmissions for use in bicycles and motor vehicles, and more particularly with the incorporation of an infinitely adjustable, variable speed transmission unit into a bicycle driving mechanism or an automotive transmission system.
b) Brief Description of the Prior Art
Most power sources such as electric motors or internal combustion engines are designed to operate at fixed rotational speeds or within a limited speed range. However, the final application of the driving power may be at a different speed or may require a range of speeds. This is normally accomplished by the use of a gear reducer with fixed elements and a fixed output speed, or, in the case of an automotive drive, a gear box with several fixed gear combinations (usually 4 or 5) which allow for several steps in speed reduction.
Different devices have already been proposed, which provide speed adjustment over a smooth curve between the limits required for the driven equipment, thereby providing for the most efficient use of the applied power, and perfect control.
A first speed adjusting device of the above mentioned type is presently in use, and comprises variable pitch pulleys connected by an endless belt. This first device is efficient but has a limited range of speed variation.
A second speed adjusting device of the above mentioned type which is also presently in use, comprises friction drives whose respective positions can be varied to vary the speed. This device is also operative but friction drives are known to be wasteful of power and not capable of transmitting large loads.
A third speed adjusting device of the above mentioned type which is also in use, consist of a hydraulic transmission comprising a hydraulic pump and a motor, or a hydraulic clutch, in which large amounts of power are wasted in fluid friction to obtain the benefits of speed control.
A fourth speed adjusting device of the above mentioned type, which, to the Applicant's knowledge, is presently not in use anywhere, has also been suggested in some patents, including, by way of examples, U.S. Pat. Nos. 2,745,297 (ANDRUS); 2,755,683 (RYAN); 3,251,243 (KRESS); 3,503,279 (SIEVERT et al); 3,861,485 (BUSCH); 4,599,916 (HIROSAWA); 4,546,673 (SHIGEMATSU et al); 4,644,820 (MACEY et al) and 4,672,861 (LANZER).
The speed adjusting devices disclosed in these patents each comprise a first shaft, a sun gear fixed to the first shaft, at least two planetary pinions meshed with the sun gear, these pinions being freely mounted on spindles forming part of a pinion carrier, and a second shaft keyed to a pinion carrier, this second shaft being coaxial with the first one.
In addition to this basic structure, the variable speed adjusting devices disclosed in the above patents each comprise a ring gear freely mounted on any of the first and second shafts, this ring gear extending over and meshing with the planetary pinions. This ring gear is operatively connected to a countershaft pinion keyed to a countershaft which extends parallel to the first shaft, and a speed variating mechanism which can be of any type, is mounted between the first shaft and the countershaft to adjustably vary the relative speed of the countershaft and first shaft.
When, on the one hand, the ring gear is directly meshed with the countershaft pinion or is operatively connected thereto through such an even number of intermediate gears and/or belts so that they rotate in opposite directions, the entire output power or any part of it that is "diverted" to the ring gear through the planetary gears, is fed back to the first shaft via the countershaft and the speed variating mechanism. As a result, any adjustment of the speed variating mechanism to cause the countershaft to rotate at a given speed or no speed relative to the first shaft provides a corresponding positive adjustment of the speed of the first shaft relative to the speed of the second shaft. Assuming that the power input shaft is the first one and this first shaft is driven at constant speed, the speed of the output shaft, i.e. the second shaft, will thus be adjustable from a predetermined maximum value in one rotational direction to zero and even to another predetermined maximum value in the other direction (see, for example, U.S. Pat. No. 2,745,297 to ANDRUS).
When, on the other hand, the control gear is operatively connected to the countershaft through one or any odd number of intermediate gears and/or high torque drive belts or chains so that they rotate in the same direction, the input power is split into two streams merging on the pinion carrier and the second shaft, one "passing" through the countershaft and the ring gear, the other directly through the sun gear. As a result, less power is transmitted through the speed variating mechanism, thereby making it possible to achieve the same functions as any conventional synchro-mesh gear box with a similar, large power input but a smaller speed variator power load and a much smaller number of parts and gears.
In both cases, the speed variating mechanism may comprise a pair of conical pulleys respectively mounted onto the drive shaft and countershaft in a head-to-foot position, with an endless belt frictionally mounted onto the pulleys. Sliding of the belt over the length of the conical pulleys permits to obtain the requested adjustment (see U.S. Pat. No. 2,755,683 to RYAN).
More preferably however, the speed variating mechanism may comprise a pair of pulleys respectively mounted onto the drive shaft and the countershaft in such a manner as to extend in the same plane, with at least one of the pulleys consisting of a variable pitch sheave, and an endless belt mounted onto the pulleys. Adjustment of the pitch of one or both of the variable sheaves permits to obtain the requested adjustment of the relative speed of the first shaft and countershaft (see, for example, U.S. Pat. No. 2,745,297 to ANDRUS, and U.S. Pat. No. 3,503,279 to SIEVERT et al).
The speed variating mechanism may further consist of a hydraulic variable speed drive or a standard, variable motor (see, for example, U.S. Pat. No. 4,546,673 to SHIGEMATSU et al).
Although the above mentioned patents clearly show that different devices of the fourth type disclosed hereinabove having different structures and/or configurations of planetary gears and variators, have been proposed in the past, none of these devices is presently in commercial use in transmission systems, essentially because none of them has met the basic requirements for any speed variating device to be useful in a transmission, namely to be capable of varying speed with the variator operating within a practical (i.e. minimum) range, and to be mechanically efficient.
To the Applicant's opinion, this lack of practicality and efficiency of the known devices of the fourth type disclosed hereinabove comes from the general belief that in such devices, whatever be their "design", a portion of the power flows from the driving means (i.e. motor or engine) through the drive shaft, sun gear and planetary gears, and another portion flows through the variator to the countershaft, ring gear, and planetary gears, both of these portions recombining to form the output power to the output shaft. In practice, this is not true and this belief is clearly wrong in the case of the device shown in the ANDRUS U.S. Pat. No. 2,745,297, where the control gear is directly connected to the countershaft pinion. Indeed, in such a case, all the power is transmitted through the sun gear to the planetary gears, and the portion of this power (which can be designated as circulating power) is transmitted to the ring gear and from the countershaft to the variator, and thence back to the main drive shaft and sun gear. The power then transmitted by the sun gear to the planetary gears is thus equal to the sum of the output power plus the circulating power plus the mechanical losses in the system.
This particularity makes the system efficiency very critical, and unless all of the structural components of the system are mathematically proportioned to minimize the mechanical losses, the overall efficiency of power transfer is too low to be acceptable for practical use. This particularity is of course critical to the design of any practical system and has been ignored in the early proposals to commercialize controlled planetary transmission systems.
The Applicant has investigated these problems and found by thorough mathematical analysis that, in order to overcome the deficiencies of the earlier designs, it is compulsory that the ratio of the radius of the sun gear to the radius of the planetary gears be kept within a very specific range. The basic formula for speed relationship between various elements of the unit is: ##EQU1## wherein N.sub.d is the rotational speed of the drive shaft (rpm);
N.sub.R is the rotational speed of the ring gear (rpm); PA1 N.sub.A is the rotational speed of the output shaft (rpm) PA1 r.sub.p is the pitch radius of the planetary gear or pinion (ft); PA1 r.sub.d is the pitch radius of the sun gear (ft); PA1 r.sub.A is the distance from the center of the drive shaft to the center of the planetary gears (ft); and PA1 r.sub.R is the pitch radius of the ring gear (internal).
The basic formula for the calculation of the circulating power, viz. the power which must be transmitted by the variator is: ##EQU2##
The circulating power can be several times as great as the transmitted power, depending on output speed and torque. As this power must be transmitted by belt, and the maximum circulating power occurs at the minimum output speed, which is when the variator is in the position where the pulley diameter is the smallest, it is essential to design the system with the minimum variation in the pulley diameters over the chosen range of speed. This is essential if the power is to be transmitted without belt slip, or breakage. If one plots various values of r.sub.p /r.sub.d versus N.sub.d /n.sub.R (which is the ratio of the drive shaft speed to the ring gear (or countershaft speed) which is essentially the variator pulley ratio, one can find that the lower is the ratio r.sub.p /r.sub.d, the smaller is the variator ratio, which gives the best power transmission possibilities. Since as demonstrated hereinabove the variator is affected by the ring gear speed ratios, the necessity to retain the lowest practical ratio r.sub.p /r.sub.d is evident, as this gives the minimum change in ring gear speeds and variator ratio.
In practice, it has been found that a good practical value for r.sub.p /r.sub.d considering the problem of a practical gear diameter to accommodate the countershaft is 0.5, which gives a variator ratio of 2:1. This has been found as the most suitable design condition to allow for transmission of maximum power. Lower r.sub.p /r.sub.d ratios as low as 0.3 may however be used for smaller power loads.
In investigating these problems, the Applicant has also found that another feature which is very important in practical design is the diameter of the ring gear which governs the overall size of the unit.
By way of example, the device disclosed in the ANDRUS U.S. Pat. No. 2,745,297 used an internal ring gear with an external gear on the outside of the internal gear, this external gear acting as control gear and meshing with the countershaft pinion. This increases the overall diameter of the unit, and by fixing the size of the external gear, does not allow any flexibility in the design of the connection between ring gear and countershaft (since it fixes the distance between the two).
An easy way of solving this problem, which is used in the present invention, consists in using a control gear which is separate from, and yet rigidly connected to the ring gear. The control gear diameter and width may be as large or small as desired, and provides complete flexibility for the design of the connection to the countershaft, and for the distance between the two shafts, thereby making it possible to accommodate any practical design for the variator.